Thanks guys, I think it is a Nano version issue. I have picked up the I2C signals sent to the SI5351 on the scope and have a look at the following photo's:
Left is from the new Nano, right is from the old Nano. It seems that the new Nano sends more instructions to the SI5351 than the old one. SI5351 recognizes both commands. There could be something in the SI5351 library which is different for different Nano versions.?
Don't know if it makes the difference in clicks, but it is a lead.
Alex
